SHINERS in cultural heritage: Can SHINERS spectra always be compared with normal Raman spectra? A study of alizarin and its adsorption in the silicon dioxide shell

Raman microspectroscopy is a powerful analytical technique in cultural heritage. However, results are often compromised by luminescence interference and poor sensitivity. Surface‐enhanced Raman spectroscopy (SERS) is an alternative that can enhance the Raman signal and quench luminescence, but the interaction of an analyte and metal substrate produces altered Raman spectra. Shell‐isolated nanoparticle‐enhanced Raman spectroscopy (SHINERS) is a promising modification of SERS that encases the metal nanoparticles in ultrathin inert shells. As a consequence, it is reported that SHINERS spectra can be directly compared with normal Raman spectra, but this is not always the case, as discussed in this work. SHINERS, SERS, and normal Raman spectra were obtained from alizarin. The resulting spectra showed that the SHINERS data are comparable with the normal Raman spectrum of alizarin adsorbed in silica, not that of isolated alizarin. The structure of alizarin adsorbed in silica is discussed. Luminescence quenching observed in the SHINERS spectra can be attributed both to LSPR of the metallic nanoparticle cores and to alizarin–silica shell chemisorption. Both silica‐encapsulated silver and gold nanoparticles were considered, and it is shown that SHINERS spectra are dependent on the interaction of alizarin with silica, rather than the specific metal core within, in contrast to our observations of Ag and Au SERS nanoparticles. The metal agnostic quality of SHINERS is a potential advantage for the collection and comparison of reference data. Additionally, SHINERS systems result in quite stable spectra in pHs ranging from, at least, 6.0 to 9.3. Future research directions for SHINERS in cultural heritage are also discussed. The resulting SHINERS spectra are comparable with normal Raman spectrum of alizarin adsorbed in silica, not that of isolated alizarin. Both Ag@SiO2 and Au@SiO2 were considered, and their spectra are dependent on the interaction of alizarin with silica, rather than the specific metal core within. Such metal agnostic quality is a potential advantage for the collection and comparison of reference data, and SHINERS systems also result in stable spectra in pHs ranging from, at least, 6.0 to 9.3, for alizarin.